1. Field of the Invention
The invention relates to a sludge extraction system, in particular for the extraction of scum in a sedimentation basin. Furthermore, the invention relates to a process for selecting the sludge partitions in accordance with their sedimentation properties in a sedimentation basin.
2. Prior Art
Mixtures of water and light materials, like oil or fat, exhibit a layer of scum on the surface of the water. Even in the biological treatment of sewage significant layers of scum can form in the presence of specific bacteria. In so doing, the air bubbles, fed in for sewage aeration, bind with the biological sludge and result in a viscous layer of scum that floats on the surface of the water.
In many areas these layers of scum cause problems, because they coat the basin walls, cause odors and a layer of ice to form, and in extreme cases the scum can spill over the basin crest. In addition, the targeted removal of the light materials is often desirable, since it facilitates the subsequent treatment. The scum is to be separated off in such a manner that large quantities of water are not simultaneously extracted.
There exist a plurality of different systems for extracting the layers of scum. In so doing, the problem is not so much the extraction of the layer of scum itself as rather the problem of conveying the light material to be extracted to the site of extraction.
The extraction systems are frequently stationarily installed. They remove the scum in the immediate vicinity of the extraction point, as desired, but the scum that is somewhat further away remains unaffected.
In the secondary sedimentation basin of sewage treatment plants, floating scum extraction systems are combined with the scrapers in the secondary sedimentation basin. In so doing, the floating scum extraction system reaches the entire basin surface in each scraping step. Since, however, the scraping motion, has to be very slow, the relative motion between the scum and the scraper is often inadequate to move the scum into the extraction system. In addition, mechanical devices must then convey the scum into channels and funnels.
The object of the invention is to provide a sludge extraction system and a process for selecting sludge partitions that allow light-weight sludge partitions to be extracted selectively and over a large area.
This problem is solved with a sludge extraction that comprises a float; a mixing unit to generate a fluid flow away from the sludge extraction; a catch tank with an overflow rim, which runs partially or completely around the sludge extraction; and a conveyor, whereby the conveying unit conveys the sludge, flowing into the catch tank, through an outlet line.
The float causes the sludge extraction to float to the surface of the water, thus extracting the floating scum and the sludge near the surface. The mixing unit generates a fluid flow away from the sludge extraction. This results in the formation of a convection current in the sedimentation basin, which on the surface of the water results in a flow from all sides in the direction of the sludge extraction. Owing to this flow all of the scum of the surface layer moves in the direction of the sludge extraction and can be collected there. In contrast to the prior art sludge extraction systems, it is possible to remove the scum and the light-weight sludge components not only locally but over a large over.
To collect the sludge there is catch tank exhibiting an overflow rim that runs partially or completely around the sludge extraction. The scum to be moved in the direction of the sludge extraction flows in free overfall over the overflow rim into the catch tank. From there it is drained through the outlet line by means of a conveyor. The overflow rim prevents the quantities of water, extracted together with the scum, from getting too large. It is possible with the compact sludge extraction of the invention to remove the layers of scum and other light-weight sludge partitions selectively and over large areas.
According to an advantageous embodiment of the invention, the catch tank is designed as a catch channel that runs completely or partially around the float.
Owing to the generated flow, the scum moves from all sides in the direction of the sludge extraction. Therefore, it is advantageous to be able to collect the sludge from as large an angular area as possible around the sludge extraction. With the aid of a catch channel, which runs completely or partially around the float, the bulk of the sludge is collected. At the same time it enables a compact design of the sludge extraction.
It is advantageous for the height of the overflow rim to be adjustable relative to the water level.
The deeper the overflow rim is positioned, the higher is the percentage of water extracted together with the scum. At the same time, however, it can be guaranteed that the complete layer of scum is extracted. At the same time, however, the pump capacity demanded of the conveying unit must be taken into consideration, because the hydraulic capacity of the extraction device must always be greater than the quantity of incoming scum. It is possible to selectively remove the scum through a suitable adjustable of the overflow rim, for example, with the aid of spindle screws.
Furthermore, it is advantageous for the catch tank to exhibit an incline, whereby the sludge, flowing into the catch tank, moves along the incline in the direction of the conveying unit.
The sludge, which falls from all sides here into the catch tank or the catch channel, can be conveyed to an extraction funnel. In this manner all of the sludge that is falling in can be pumped off by means of the conveying unit.
According to another advantageous embodiment of the invention, the conveying unit is a submersible pump. Such submersible pumps are designed for use under water; and, therefore, such a submersible pump can be mounted below the catch channel. This option of being able to mount the pump underneath the water level enables a compact design of the sludge extraction.
According to another advantageous embodiment of the invention, the mixing unit comprises an agitator, with which the fluid flow, directed away from the sludge extraction, can be generated.
Such an agitator usually comprises a propeller, whose drive shaft is driven by an (electric) motor. Whereas the design of the propeller determines the flow profile generated, the rate of flow can be set by means of the speed of the drive motor. Such an agitator constitutes the simplest and least expensive possibility of generating the desired fluid flow.
Another advantageous embodiment of the invention provides a frequency converter, with which the speed of the agitator can be set.
The higher the speed of the propeller is chosen, the stronger is the convection current generated in the sedimentation basin. Whereas with a weak convection current only the layer of scum and light-weight sludge particles in the vicinity of the water surface can be extracted, a stronger convection current can bring about that also heavier sludge particles are swirled up so as to form layers near the surface. Therefore, as the speed of the agitator increases, higher densities of sludge partitions can also be extracted. In this manner it is possible to determine the composition of the sludge extracted by the rotational frequency of the agitator.
This is especially significant if the sludge extraction of the invention is used in biological sewage clarification, and in particular in activation tanks. In the activation tank, high molecular sewage particles are decomposed with the aid of bacteria. These bacteria need oxygen; and, therefore, the activation tank must be sufficiently aerated. The biomass in the activation tank is called the socalled xe2x80x9cactivated sludgexe2x80x9d. Since the bacteria are constantly multiplying, the mass of activated sludge also increases. Therefore, not only the scum but also a part of the activated sludge is also being continuously extracted from the activation tank. When the sludge extraction of the invention is used in an activation tank, one can determine through the choice of the speed of the agitator to what extent not only the scum but also the lighter weight components of the activated sludge are also being extracted.
A preferred embodiment of the present invention provides means for fixing the float in a horizontal position. The components of the mixing unit and in particular the submersible pump ensure uniform loading of the float. The result of this state is that the sludge extraction lies obliquely in the water. However, it is necessary for the sludge extraction to function that the height of the overflow rim be constant over the periphery with respect to the water surface. Hence it is advantageous to provide means that fix the sludge extraction in a horizontal plane.
There can be two parallel guide elements, which are pivot mounted on the sludge extraction and on a fastening device, whereby the distance between the swivel axes on the sludge extraction and on the fastening device is identical, and whereby the connecting lines of the swivel axes run parallel on the sludge extraction and on the fastening device. The guide elements are configured in the sense of a parallelogram, thus guaranteeing that the horizontal alignment of the sludge extraction and the float is maintained independently of the height of the water level.
It is especially advantageous for at least one of the guide elements to be formed by the outlet line. In this manner an especially simple and inexpensive design is obtained.
Another embodiment of the invention provides that at least one of the guide elements is hinged at two points respectively on the sludge extraction and on the fastening device. The wall sided positioning at two points and the corresponding counter positioning at the float prevent the sludge extraction from moving sideways should a capsizing instance occur. It is thus guaranteed that the position of the sludge extraction in the water is stable.
The described sludge extraction can be used, on the one hand, to simultaneously extract scum and, on the other hand, sludge partitions can be selected according to their sedimentation properties with such an extraction. The stronger the generated convection current is chosen, the higher is the percentage of removed heavy sludge particles. Inversely if the current is weak, only the lightest sludge particles can be extracted.
The process of the invention for selecting sludge partitions in accordance with their sedimentation properties in a sedimentation tank comprises the following steps. A fluid flow is generated in the sedimentation tank. By means of a sludge extraction, which exhibits an overflow rim just below the water level, a sludge partition exhibiting the predetermined sedimentation properties is collected and then carried away.
In contrast to the extraction devices described in the prior art, it is possible to extract the scum and the lighter weight sludge partitions over a large area with the aid of the fluid flow generated in the sedimentation tank. In so doing, only sludge particles having specific sedimentation properties are collected and carried away. The collected sludge partition can be affected by varying the generated fluid flow, especially by varying the rate of this flow.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.